Air-laid structures are widely used in the art of absorbent articles and other arts in which fibrous webs are of use. One common approach for creating air-laid structures is to process a fibrous sheet of cellulosic fibers or other suitable fibers through a device that breaks up the fibrous sheet, thereby forming discrete fibers. The discrete fibers are entrained in a stream of air and directed to a foraminous forming surface upon which the fibers are deposited to form fluff. Typically, fluff has a high porosity and is comprised of essentially randomly oriented fibers. In some processes, a vacuum is applied to one side of the foraminous surface to create a pressure differential across the foraminous forming surface to assist with drawing the discrete fibers to the foraminous forming surface.
Absorbent articles such as sanitary napkins, diapers, and adult incontinence products commonly employ air-laid structures in the absorbent core. Absorbent cores have a generally planar structure in which the thickness is generally smaller than the planar dimensions. One common approach to forming air-laid absorbent articles is to situate the foraminous surface in a recess. In the art, the structure in which the foraminous surface is emplaced and the foraminous surface are components of what is commonly referred to as a core pocket. The thickness can be partially controlled by the depth of the recess in the core pocket and the planar dimensions of the absorbent core can be defined by the dimensions of the recess and the foraminous surface
Absorbent cores having a variable thickness are thought to fit wearers better and to have better fluid acquisition properties. Absorbent cores in which the thickness of the absorbent core varies can be created by using a foraminous surface that is contoured in the thickness dimension. Thus, different quantities of fibers are required to fill different portions of the core pocket. One problem with filling a contoured foraminous surface to form an absorbent core having variable thickness is that a substantial amount of scarfing may need to be performed on the core to render the core flat on the side of the pad opposing the contoured side, which is the typical construction of many types of absorbent cores. For absorbent cores such as those used in sanitary napkins, the contoured side can be the body facing surface and a flat garment facing side can be desired or vice versa.
During formation of the article, the fibers filling a core pocket having a foraminous surface that is contoured in the thickness dimension can be thought of as being like snow falling in a ditch. When enough snow has fallen such that the level of snow in the deepest portion of the ditch is even with the level of the ground surrounding the ditch, the level of the snow above the shallower edges of the ditch will be above the level of the surface of the ground surrounding the ditch. Thus, to make the level of snow in the ditch even or flat, snow above the shallower edges of the ditch needs to be scarfed away to be even with the level of snow in the deepest portion of the ditch. An analogous situation arises in air laying fibrous articles having a contoured thickness in which one surface is desired to be flat.
Scarfing imparts mechanical energy to the fibrous articles which can result in uncontrollable changes in the basis weight and structure of the fibrous articles. Many designers of absorbent articles recognize that small variations in the basis weight and structure of absorbent cores can have significant impacts on the fluid acquisition and retention properties of absorbent articles. Uncontrolled variations in basis weight within a single absorbent core and uncontrolled variations in basis weight between multiple absorbent cores formed on a single manufacturing line can be unacceptable to manufacturers of absorbent articles because consumers demand that absorbent articles sold under a particular brand name perform consistently. Furthermore, scarfing can result in wasted fibrous material and if the scarfed fibrous material is recycled, some of the fibers can be damaged by scarfing.
Absorbent cores having a basis weight that varies in plane can be desirable. The partially saturated fluid acquisition and retention properties of air-laid absorbent cores are known to vary as a function of basis weight. For instance, some designers of absorbent articles designed to be worn close to the human body desire an absorbent core in which the basis weight of the center of the absorbent core is greater than the basis weight of the periphery of the absorbent core. For some designs of absorbent articles, precise variations in basis weight in the machine direction and cross direction can be helpful with achieving optimum performance of the absorbent article.
With these limitations in mind, the problem remains with providing an apparatus to manufacture air-laid fibrous articles in which the basis weight can vary in plane. There is a further unmet need for providing an apparatus in which the variation in the basis weight in the machine direction and cross direction can be precisely controlled. There is an additional unmet need for an apparatus for forming air-laid fibrous articles in which the amount of excess fibrous material deposited is minimized.